A liposome is a closed generally spherical vesicle with a membrane composed of a phospholipid bilayer that is capable of encapsulating water-soluble, hydrophilic molecules in their aqueous core and embedding oil-soluble, hydrophobic molecules in the hydrophobic region of the bilayer. Liposomes typically are small particles, between about 25 nm to 1000 nm and composed of a variety of phospholipids, including naturally-derived phospholipids with mixed lipid chains, such as egg phosphatidylethanolamine, or of pure components like DOPE (dioleolylphosphatidylethanolamine). Liposomes are widely used in topical drug preparations, by providing a carrier mechanism for deposition of active compounds into the intrinsic layers of the skin. Benefits of employing this technology include controlled release, selective delivery, enhanced bioavailability and stability, and increased absorption of desired actives. In addition, liposomes have a great affinity and compatibility with the skin due to their chemical resemblance to the extracellular lipids surrounding internal skin cells. The lipid bilayer of a liposome can fuse with other bilayers, for example cellular membranes, thus delivering the liposome contents.
The commercial development of liposomes has been plagued by poor stability, clarity, and short shelf life. Liposome vesicles tend to fuse together or agglomerate, particularly when they are exposed to surfactants, solvents, adverse pH conditions, elevated temperatures, or even water, for long periods of time.